Liquid membranes combine extraction and stripping, which are normally carried out in two separate steps in conventional processes such as solvent extractions, into one step. A one-step liquid membrane process provides the maximum driving force for the separation of a targeted species, leading to the best clean-up and recovery of the species (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992).
There are two types of liquid membranes: (1) supported liquid membranes (SLMs) and (2) emulsion liquid membranes (ELMs). In SLMs, the liquid membrane phase is the organic liquid imbedded in pores of a microporous support, e.g., microporous polypropylene hollow fibers (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992). When the organic liquid contacts the microporous support, it readily wets the pores of the support, and the SLM is formed. ELMs are usually prepared by first forming an emulsion between two immiscible phases, and then dispersing the emulsion in a third (continuous) phase by agitation for extraction. The membrane phase is the liquid phase that separates the encapsulated, internal droplets in the emulsion from the external, continuous phase (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992).
The use of liquid membranes to remove chromium from wastewaters has long been pursued by the scientific and industrial community (W. S. Winston Ho and Kamalesh K. Sirkar, eds., Membrane Handbook, Chapman & Hall, New York, 1992; J. W. Frankenfeld and N. N. Li in N. N. Li, ed., Recent Developments in Separation Science, pp. 285-292, CRC Press, Boca Raton, Fla., 1977; E. J. Fuller and N. N. Li, J. Membrane Sci., 18, 251 (1984); A. I. Alonso and C. C. Pantelides, J. Membrane Sci., 110, 151 (1996)). Nonetheless, there remains a need for a process that not only can remove chromium but also can recover it.
The chromium recovered in high concentration is desirable for reuse or resale. For example, a highly concentrated chromium solution containing at least 15 wt. % hexavalent chromium (Cr(VI)), e.g., 46.7 wt. % Na.sub.2 CrO.sub.4, and less than 10 wt. % Na.sub.2 SO.sub.4 is a useful form of a product for reuse or resale.
It is an object of the present invention to provide a novel supported liquid membrane process that not only removes chromium to an acceptable level in the treated water suitable for discharge or recycle but also recovers the chromium in a usable form for reuse or resale. This process not only solves the environmental problem but also recovers the chromium.